Phenolic novolak resins have conventionally been used as hardeners for epoxy resins. Besides ordinary phenolic novolak resins, an epoxy resin hardener having a similar structure is disclosed in U.S. Pat. No. 3,936,510. This hardener is a synthetic resin obtained by reacting phenol with 1,4-di(methoxymethyl)benzene and having the following structure in which a phenol nucleus, methylene group, benzene nucleus, and methylene group are bonded in this order to form a repeating unit. ##STR1##
However, the synthetic resin having such a repeating unit has been defective in that use of the synthetic resin as an epoxy resin hardener gives cured epoxy resins having high water absorption and insufficient water resistance.